Osseous differentiation within a phyllodes tumor is extremely rare. Cytological and histological findings of a case of malignant phyllodes tumor with osseous differentiation are presented. A 45-year-old female had a malignant phyllodes tumor with osseous stroma diagnosed by fine needle aspiration cytology. The cytological findings were representative of the histological features. The diagnosis of these tumors preoperatively is important in planning the most appropriate treatment. It is also important to follow up these patients postoperatively for long periods for recurrence and metastasis.